Classification of Defects in CIGS Solar Cells and Modules Using Electroluminescence and Thermography Techniques

Abstract

We propose a classification of defects in large-area Cu(In,Ga)Se2 (CIGS) modules using electroluminescence (EL) and lock-in thermography (LIT) imaging techniques. We observe three types of defects, which we name according to their respective appearance in EL images as “Smears”, “Freckles”, and “Doughnuts”. “Smears” consist of a small dark spot and a tail (length: 2 to 12 mm), which smears out over several cells. In both EL and LIT images, Smears appear as shunts with negative impact on the cell performance. “Freckles” appear in EL images as dark spots with sharp contours (diameter: 10 to 200 μm). Doughnuts are visible in EL images as dark spots (diameter: 10 to 50 μm), surrounded by bright edge (diameter: 20 to 150 μm). Freckles and Doughnuts may originate from impurities in the CIGS absorber layer, and ZnO layer, respectively. Their appearances in EL and LIT images show that these two defect types have minor electrical impact and may result from difference in emissivity of materials. However, both of them cause a reduction of active area, and thus the short circuit current. Preliminary results from scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) indicate the presence of carbon and potassium containing contamination